`bevy_gizmos`: Retained text gizmos

## What problem does this solve or what need does it fill?

The layouts and line geometry for text gizmos is regenerated every frame. This isn't that much of a problem as text gizmos are mostly intended for drawing short sections of text, like labels and numbers. We could get better performance by retaining the output though, updating it only on changes to the text.

## What solution would you like?

I don't know enough about the exisiting retained gizmos support to have an informed opinion on the design.